The direct synthesis of methylchlorosilanes from silicon and methyl chloride at 250.degree. to 300.degree. C. using copper catalysts produces not only methylchlorosilanes of the general formula Me.sub.x SiCl.sub.4-x, where x is from 0 to 4, but also various hydrogensilanes and high boiling by-products, primarily methylchlorodisilanes of the general formula Me.sub.x Si.sub.2 Cl.sub.6-x, where x is from 2 to 5.
The methylchlorosilanes are separated and purified by distillation. The distillation leaves a high boiling fraction which accounts for from 5 to 10% of crude product. It is economically and ecologically desirable to further treat these residues to isolate useful materials. This is achieved by separating the distillable methylchlorodisilanes and cleaving them with hydrogen chloride into methylchlorosilanes. In general, this cleavage is carried out with amine catalysts in the liquid phase. The work-up of the residue is described inter alia in W. Noll, (Chemistry and Technology of Silicones) 2nd Edition 1968, Academic Press, Inc., Orlando, Chap. 2.2.8. The chemical properties of disilanes and the fundamental principles of cleavage are described in R. Calas et al., J. Organomet. Chem. 225 (1982) 117-130.
Usually the high boiling fraction is worked up using a batchwise distillation in order to isolate the methylchlorodisilane fraction which is cleavable with hydrogen chloride. This distillation gives rise to the following fractions:
______________________________________ Boiling range Main components ______________________________________ 70-140.degree. C. Dimethyldichlorosilane Fraction 1 Ethylmethyldichlorosilane Methylpropyldichlorosilane Ethyldimethylchlorosilane Methylchlorodisilanes Hydrocarbons 140-155.degree. C. Pentamethylchlorodisilane Fraction 2 Tetramethyldichlorodisilane Trimethyltrichlorodisilane Dimethyltetrachlorodisilane 155-160.degree. C. Trimethyltrichlorodisilane Fraction 3 Dimethyltetrachlorodisilane &gt;160.degree. C. Oligomethylsilanes, including Fraction 4 dimethyltetrachlorodisilane Carbosilanes Siloxanes Metals and metal halides ______________________________________
Only those methylchlorodisilanes which have at least two chlorine atoms attached to one silicon atom, i.e., not the symmetrical tetramethyldichlorodisilane or pentamethylchlorodisilane are cleavable with hydrogen chloride to form monosilanes having the equation ##STR1##
The cleavage product comprises essentially dimethyldichlorosilane, methyltrichlorosilane and methylchlorodisilane.
The cleavage of the cleavable disilanes of fraction 3 is conducted in reaction vessels in which it is possible to remove the cleavage product mixture, for example in a distillation flask with an attached column. The distillation removes the cleavage product silane at the top of the column, while noncleavable components of the disilane feed mixture build up in the reaction mixture and necessitate regular evacuation of the contents from the reactor.
Those fractions from the work-up of the high boiling residues which contain little if any cleavable disilanes, in particular fraction 4, as well as the disilane cleavage residue to be evacuated which still contains cleavable disilanes have to be disposed of by incineration.
Therefore, it is an object of the present invention to provide a process for separating methylchlcrosilanes from high boiling residues from the methylchlorosilane synthesis. Another object of the present invention is to provide a process for separating methylchlorosilanes from high boiling residues from the methylchlorosilanes synthesis in which the cleavable methylchlorodisilanes can be cleaved continuously to form a higher yield of silanes from the cleavage product.